andy the best thing to stop forearm pump is true try to relax when riding plus to combat this is several exersizes i.e squeezing a squash ball start with a white dot up to a red dot? whitesoft red hard
or grip tester but thats a bit extreeme
if you want strong powerful forearms get a dumbell turn your wrist so your knuckles are facing the floor lay your forearm on a bench or chair and twist towards the bicept and back towards the ground .......... but this will make you forearms muscular as well which when worked can cause a latic acid build up and cause cramp which is like what your getting?
when im ready to compete for a fight i also do a exercize with a rope and a weight and a piece of steel rode and wind the rope which pulls the wieght off the ground this gives you stupid forearm strength? now when i was motor crossing i had to learn to relax and not hold on too much allow the bars to loosly to rumble between your fingers and only hold on when hitting a table top or a burm in the bend good luck andy any help give me a shout?